This Phase I clinical trial will be the first to test the concept of a "chemosensitization" approach in children using Plerixafor. Patients aged 3 to 30 with relapsed/refractory AML, ALL or MDS will receive Plerixafor followed 4 hours later with combination chemotherapy consisting of etoposide and cytarabine daily for five days. We will determine the safety and tolerability of Plerixafor in combination with cytarabine and etoposide in pediatric and young adults with relapsed/refractory acute leukemias. The secondary objectives of this study will quantify the peripheral blood mobilization of blasts in response to Plerixafor using flow cytometry, measure initial CXCR4 expression on leukemic blasts and correlate with response, and determine the change in CXCR4 expression after protocol therapy. Finally, we will determine the pharmacokinetics of Plerixafor when administered with cytotoxic chemotherapy in this patient population.